1. Field of the Invention
The present invention relates to a rotational angle detector for detecting a rotational angle of a rotary element and a rotary machine including a rotational angle detector.
2. Description of the Related Art
A rotational angle detector is used to detect a rotational angle of a rotary machine such as an electric motor, or a rotational angle of a rotary element driven by a rotary machine. For example, JP 2006-010436 A discloses a magnetic rotational angle detector which includes a magnetic sensor for detecting the magnitude of a magnetic field varying in response to the rotational angel of a rotary element. This type of rotational angle detector includes a detected portion in the form of a gear provided on an outer circumferential face of the rotary element, and a magnetic sensor provided so as to be opposite to the detected portion.
FIG. 7 is a perspective view illustrating a rotational angle detector 100 according to related art. The rotational angle detector 100 includes a cylindrical rotary element 104 fixed to a rotational axis 102 rotatable around a rotational axis line 102a, and a detection element 108 fixed to a cylindrical seat 106. The rotary element 104 has a detected portion 104a on an outer circumferential face. The detection element 108 is positioned so as to be opposite to and close enough to the detected portion 104a of the rotary element 104.
In magnetic rotational angle detector, such as disclosed in JP 2006-010436 A, detection accuracy may be decreased when the positional relationship between the magnetic sensor incorporated in the detection element 108 and the detected portion 104a of the rotary element 104 is changed. Therefore, it is necessary to ensure that the detection element 108 is in a predetermined position relative to the detected portion 104a. 
However, the position of the detection element 108 relative to the rotary element 104 may be changed, due to accumulated dimensional tolerances of the rotational axis 102, the rotary element 104, the seat 106 and other structures, which are not shown in the drawing, or an assembly error. Therefore, as shown in FIG. 7, a spacer 110 is provided between the detection element 108 and a mounting surface 106a of the seat 106 in order to adjust the position of the detection element 108 relative to the detected portion 104a in a direction parallel to the rotational axis line 102a. 
FIG. 8 is a perspective view illustrating a rotational angle detector 100 according to another related art. In this related art, a seat 106′ having a larger thickness is used, instead of the spacer 110. For comparison, the position of the mounting surface 106a of the seat 106 having a thickness T1 as shown in FIG. 7 is shown by dashed line. The seat 106′ has a larger thickness T2 than the thickness T1. As a result, the mounting surface 106a′ of the seat 106′ and the detection element 108 fixed on the mounting surface 106a′ are at a higher position by the difference in thicknesses. By selectively adopting a plurality of seats having different thicknesses from one another, the position of the detection element 108 relative to the detected portion 104a in a direction parallel to the rotational axis line 102a can be adjusted.
However, according to these methods, in which spacers or seats having different thicknesses are selectively used to adjust the position of the detection element, it is necessary to prepare a plurality of different spaces or seats for assembly of the rotary machine. In addition, since a different spacer or seat may be required every time the rotary machine or the rotational angle detector is disassembled for repair and inspection, operational efficiency is decreased. Further, the management cost of parts is increased.
Therefore, there is a need for a rotational angle detector which ensures a predetermined positional relationship between the detected portion and the detection element, without a plurality of different spacers or seats.